In check valve assemblies employing an elastomeric umbrella valve, the latter typically has an integral stem by which it is normally attached in a preloaded position of closure and is deflectable in response to differential pressure to assume an open position permitting flow therepast in one direction only. It has been found that should the valve become unattached for some reason such as fatigue of the stem, it may be forced by the fluid pressure into a flow blockage position preventing the desired one way flow and not thereafter capable of assuming a closure position preventing flow in the opposite direction.